1. Field of the invention
The invention relates to objective lens actuators.
2. Related art
Conventionally, an optical recording and reproducing apparatus, such as an optical disk device using an optical disk as a recording medium, employs an optical pickup for writing data signals to and reading the data signal from the disk. An objective lens actuator is employed in this optical pickup to correctly converge a laser beam onto a data bit on the disk. FIGS. 6 and 7 show an example of such an objective lens actuator.
FIGS. 6 and 7 show the objective lens actuator recited in Unexamined Japanese Patent Publication No. Hei. 3-165331. As shown in FIG. 6, an acrylate resin member 140 is arranged along the entire circumference between an objective lens holding body 131 and control coils 134, 135, so that this acrylate resin member 140 can increase the damping ratio of a movable part to thereby control resonance peaks to low values.
In FIGS. 6 and 7, a circular recess 121 is formed on a base 120. An optical beam through hole 122 is formed in a part of this recess 121 so as to pass the optical beam from a light source therethrough. A mounting hole 124 for securing a support shaft 123 is arranged at the center of the recess 121. A rubber spring 125 is arranged inside the recess 121 on a side opposite to the optical beam through hole 122 with the mounting hole 124 interposed therebetween. Further, arcuate magnet holding strips 126a, 126b are arranged inside the recess 121 so as to project upward at such positions as to confront each other while interposing a line connecting the center of the optical beam through hole 122 to the center of the support shaft 123 therebetween. Similarly arcuate magnetic path strips 127a, 127b are formed on outer circumferential sides of the magnet holding strips 126a, 126b outside the recess 121.
A bearing 128 that is rotatable about and vertically movable along the axis of the support shaft 123 is fitted into the thus fixed support shaft 123. The central portion of a substantially rectangular objective lens holding body 131 is coupled to the bearing 128. An objective lens 129 is set into an upper portion of an optical beam injection hole 139 arranged in this objective lens holding body 131. The objective lens 129 is set into the objective lens holding body 131 at a position corresponding to the optical beam through hole 122, and a balancer 130 is set into the objective lens holding body 131 at a position corresponding to the rubber spring 125. A holding pin 132 provided at the bottom of the balancer 130 is coupled to the rubber spring 125. The coupling of the holding pin 132 to the rubber spring 125 allows the objective lens holding body 131 to return to a predetermined position at all times every time the objective lens holding body 131 rotates or moves vertically. Arcuate magnets 133a, 133b are secured to the outer circumferences of the magnet holding strips 126a, 126b, respectively. A gap is provided between the outer circumferences of the magnets 133a, 133b and the magnetic path strips 127a, 127b.
A ringlike wound focusing control coil 134 is secured to the outer circumference of the objective lens holding body 131 through the acrylate resin member 140. On the outer circumference of the focusing control coil 134 is the tracking control coil 135, which is formed by coupling four arcuately shaped coils to one another at a predetermined interval. Both coils 134 and 135 are placed in a magnetic field formed between the magnets 133a, 133b and the magnetic path strips 127a, 127b.
A cover 136 is put over the whole body of the thus set components. Windows 137, 138 are formed at positions corresponding to the objective lens 129 and the bearing 128 as well as to the balancer 130 on the upper surface of the cover 136. The window 137 functions not only as a member for transmitting an optical beam past the objective lens 129 but also as a member for preventing the bearing 128 from interfering with the cover 136 when the bearing 128 makes an upward movement together with the objective lens holding member 131.
In the aforementioned construction, an optical beam is injected from the light source, and the injected optical beam is converged on a data recording medium through the objective lens 129 so that optical beam spot displacement as well as track groove displacement on the recording medium surface are detected. Then, currents corresponding to the detected displacements are applied to the focusing control coil 134 and the tracking control coil 135, respectively, to utilize magnetic flux in the magnetic circuit formed by the magnets 133a, 133b and the magnetic path strips 127a, 127b, so that the objective lens is moved in the focusing direction as well as in the tracking direction by causing the objective lens holding body 131 to either vertically move along or rotate about the axis of the support shaft 123, the objective lens holding body having both coils being secured to the outer circumference thereof.
Since the acrylate resin member 140 is interposed between the focusing control coil 134 and the objective lens holing body 131 in this case, the damping ratio of a movable part consisting of the objective lens 129, the objective lens holding body 131, the bearing 128, the acrylate resin member 140, the focusing control coil 134, the tracking control coil 135, and the balancer 130 can be increased, so that it is not likely to increase resonance peaks at high resonant frequencies of the movable part in the focusing direction and in the tracking direction, nor is it likely to impair the stability of the servo motor.
However, the objective lens actuator disclosed in Unexamined Japanese Patent Publication No. Hei. 3-165331 has the following problems.
The acrylate resin member 140 is interposed between the magnets 133a, 133b and the magnetic path strips 127a, 127b, and the presence of the acrylate resin member 140 leads to an increase in the distance between the magnets 133a, 133b and the magnetic path strips 127a, 127b, which in turn imposes the problem of reduction in magnetic characteristics.
Further, this acrylate resin member 140 does not provide sufficient damping, and the operation of mounting the acrylate resin member 140 is cumbersome, which is another problem. This problem is likewise encountered when, e.g., an adhesive member or the like is used as a resonance damping means other than the acrylate resin member 140.